1. Field of the Invention
This invention relates to making beads of expandable polystyrene, in particular to the making of beads destined for use in making containers such as drinking cups. More particularly, it relates to a washing procedure which seems to overcome mold-filling and static problems.
2. Description of the Prior Art
It may be taken as known that styrene monomer may be polymerized together with a free-radical initiator and water and polyvinyl pyrrolidone and pentane to produce beads of expandable polystyrene containing a suitable proportion of pentane as blowing agent and having a desired particle size, such as one on the order of 0.4 to 0.6 millimeters. Those skilled in the art are familiar with the techniques required for obtaining a product having particles of the desired degree of polymerization and the desired size, adjusting such parameters as the nature and concentration of the free-radical initiator used, the time-temperature cycle employed, the severity of the agitation provided during the suspension-polymerization process, and the amount and the time of addition of the polyvinyl pyrrolidone. Although the practice, using polyvinyl pyrrolidone as one of the ingredients in the recipe, has proved satisfactory for the making of expandable-polystyrene beads which are used for insulation, there had not been known, prior to this invention, how such a recipe, using polyvinyl pyrrolidone, could be used to make an expandable-polystyrene bead product which would be suitable for use in the making of drinking cups or other impervious containers. Previous efforts to make expandable-polystyrene beads from such a recipe for such a use met with failure, in that too great a percentage of the product containers exhibited leaking. Moreover, the containers, after being formed, even if they did not exhibit leaking, exhibited undesirable static-electricity effects, tending to attract or to repel one another and making their handling difficult.